Memory pipeline control in a hierarchical memory system

ABSTRACT

In described examples, a processor system includes a processor core generating memory transactions, a lower level cache memory with a lower memory controller, and a higher level cache memory with a higher memory controller having a memory pipeline. The higher memory controller is connected to the lower memory controller by a bypass path that skips the memory pipeline. The higher memory controller: determines whether a memory transaction is a bypass write, which is a memory write request indicated not to result in a corresponding write being directed to the higher level cache memory; if the memory transaction is determined a bypass write, determines whether a memory transaction that prevents passing is in the memory pipeline; and if no transaction that prevents passing is determined to be in the memory pipeline, sends the memory transaction to the lower memory controller using the bypass path.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of and priority to U.S. Provisional Patent Application Ser. No. 62/852,480, filed May 24, 2019, entitled WRITE STREAMING IN A PROCESSOR, which is hereby fully incorporated herein by reference.

TECHNICAL FIELD

The present disclosure relates generally to a processing device that can be formed as part of an integrated circuit, such as a system on a chip (SoC). More specifically, this disclosure relates to such a system with improved management of write operations.

BACKGROUND

An SOC is an integrated circuit with multiple functional blocks (such as one or more processor cores, memory, and input and output) on a single die.

Hierarchical memory moves data and instructions between memory blocks with different read/write response times for respective processor cores, such as a central processing unit (CPU) or a digital signal processor (DSP). For example, memories which are more local to respective processor cores will typically have lower response times. Hierarchical memories include cache memory systems with multiple levels (such as L1 and L2), in which different levels describe different degrees of locality or different average response times of the cache memories to respective processor cores. Herein, the more local or lower response time cache memory (such as an L1 cache) is referred to as being a higher level cache memory than a less local or higher response time lower level cache memory (such as an L2 cache or L3 cache). Associativity of a cache refers to the cache storage segregation, where set associativity divides the cache into a number of storage sets and each such set stores a number (the way) of blocks, while a fully associative cache is unconstrained by a set limitation. Accordingly, for an integer N, each location in main memory (system memory) can reside in any one of N possible locations in an N-way associative cache.

A “victim cache” memory caches data (such as a cache line) that was evicted from a cache memory, such as an L1 cache. If an L1 cache read results in a miss (the data corresponding to a portion of main memory is not stored in the L1 cache), then a lookup occurs in the victim cache. If the victim cache lookup results in a hit (the data corresponding to the requested memory address is present in the victim cache), the contents of the victim cache location producing the hit, and the contents of a corresponding location in the respective cache (L1 cache in this example), are swapped. Some example victim caches are fully associative. Data corresponding to any location in main memory can be mapped to (stored in) any location in a fully associative cache.

SUMMARY

In described examples, a processor system includes a processor core generating memory transactions, a lower level cache memory with a lower memory controller, and a higher level cache memory with a higher memory controller having a memory pipeline. The higher memory controller is connected to the lower memory controller by a bypass path that skips the memory pipeline. The higher memory controller: determines whether a memory transaction is a bypass write, which is a memory write request indicated not to result in a corresponding write being directed to the higher level cache memory; if the memory transaction is determined a bypass write, determines whether a memory transaction that prevents passing is in the memory pipeline; and if no transaction that prevents passing is determined to be in the memory pipeline, sends the memory transaction to the lower memory controller using the bypass path.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an example processor that is a portion of an SoC.

FIG. 2 is a block diagram of an example memory pipeline for the SoC of FIG. 1.

FIG. 3 is an example of a process for memory write operations for the SoC of FIG. 1.

FIG. 4 is a block diagram of an example memory pipeline for the SoC of FIG. 1.

FIG. 5 is an example of a process for memory write operations for the SoC of FIG. 1.

DETAILED DESCRIPTION

FIG. 1 is a block diagram of an example processor 100 that is a portion of an SoC 10. SoC 10 includes a processor core 102, such as a CPU or DSP, that generates new data. Processor 100 can include a clock 103, which can be part of processor core 102 or separate therefrom (separate clock not shown). Processor core 102 also generates memory read requests that request reads from, as well as memory write requests that request writes to, a data memory controller 104 (DMC) and a streaming engine 106. In some embodiments, processor core 102 generates one read request or write request per processor core clock cycle. Memory controllers return a write acknowledgment to processor core 102 to confirm that a requested memory write has been performed. Processor core 102 is also coupled to receive instructions from a program memory controller 108 (PMC). Streaming engine 106 facilitates processor core 102 sending certain memory transactions and other memory-related messages that bypass DMC 104 and PMC 108.

SoC 10 has a hierarchical memory system. Each cache at each level may be unified or divided into separate data and program caches. For example, the DMC 104 may be coupled to a level 1 data cache 110 (L1D cache) to control data writes to and data reads from the L1D cache 110. Similarly, the PMC 108 may be coupled to a level 1 program cache 112 (L1P cache) to read instructions for execution by processor core 102 from the L1P cache 112. (In this example, processor core 102 does not generate writes to L1P cache 112.) The L1D cache 110 can have an L1D victim cache 113. A unified memory controller 114 (UMC) for a level 2 cache (L2 cache 116, such as L2 SRAM) is communicatively coupled to receive read and write memory access requests from DMC 104 and PMC 108, and to receive read requests from streaming engine 106, PMC 108, and a memory management unit 117 (MMU). (The example L2 controller UMC 114 is called a “unified” memory controller in the example system because UMC 114 can store both instructions and data in L2 cache 116.) UMC 114 is communicatively coupled to pass read data and write acknowledgments (from beyond level 1 caching) to DMC 104, streaming engine 106, and PMC 108, which are then passed on to processor core 102. UMC 114 is also coupled to control writes to, and reads from, L2 cache 116, and to pass memory access requests to a level 3 cache controller 118 (L3 controller). L3 controller 118 is coupled to control writes to, and reads from, L3 cache 119. UMC 114 is coupled to receive write acknowledgments, and data, read from L2 cache 116 and L3 cache 119 (via L3 controller 118). UMC 114 is configured to control pipelining of memory transactions for program content and data content (read and write requests for instructions, data transmissions, and write acknowledgments). L3 controller 118 is coupled to control writes to, and reads from, L3 cache 119, and to mediate transactions with exterior functions 120 that are exterior to processor 100, such as other processor cores, peripheral functions of the SOC 10, and/or other SoCs (and also to control snoop transactions). That is, L3 controller 118 is a shared memory controller of the SoC 10, and L3 cache 119 is a shared cache memory of the SoC 10. Accordingly, memory transactions relating to processor 100 and exterior functions 120 pass through L3 controller 118.

Memory transactions are generated by processor core 102 and are communicated towards lower level cache memory, or are generated by exterior functions 120 and communicated towards higher level cache memory. For example, a victim write transaction may be originated by UMC 114 in response to a read transaction from the processor core 102 that produces a miss in L2 cache 116.

MMU 117 provides address translation and memory attribute information to the processor core 102. It does this by looking up information in tables that are stored in memory (connection between MMU 117 and UMC 114 enables MMU 117 to use read requests to access memory containing the tables).

FIG. 2 is a block diagram of an example memory pipeline 200 included within or associated with the FIG. 1 UMC 114, so for illustration FIG. 2 also repeats various blocks from FIG. 1 that communicate with UMC 114. Memory pipeline 200 includes an initial scheduling block 202 coupled to an integer number M of pipeline banks 206. Each pipeline bank 206 includes an integer number P of stages 208 and is illustrated as a vertical column below initial scheduling block 202. DMC 104 is coupled to initial scheduling block 202 by a bus 204-1 that is a number N₁ lines wide, enabling DMC 104 to make a read or write request transferring a number N₁ bits of data at a time. Streaming engine 106 is coupled to initial scheduling block 202 by a bus 204-2 that is a number N₂ lines wide, enabling streaming engine 106 to make a read request transferring a number N₂ bits of data at a time. PMC 108 is coupled to initial scheduling block 202 by a bus 204-3 that is a number N₃ lines wide, enabling PMC 108 to make a read request transferring a number N₃ bits of data at a time. L3 controller 118 is coupled to initial scheduling block 202 by a bus 204-4 that is a number N₄ lines wide, enabling L3 118 to make a read or write request transferring a number N₄ bits of data at a time. MMU 117 is coupled to initial scheduling block 202 by a bus 204-5 that is a number N₅ lines wide, enabling MMU 117 to make a read request transferring a number N₅ bits of data at a time.

When a memory controller of processor 100 (such as DMC 104, streaming engine 106, PMC 108, MMU 117, or L3 controller 118) communicates to UMC 114 a request for a read from, or a write to, a memory intermediated by UMC 114 (such as L2 cache 116, L3 cache 119, or a memory in exterior functions 120), initial scheduling block 202 schedules the request to be handled by an appropriate pipeline bank 206 for the particular request. Accordingly, initial scheduling block 202 performs arbitration on read and write requests. Arbitration determines which pipeline bank 206 will receive which of the memory transactions queued at the initial scheduling block 202, and in what order. Typically, a read or write request is scheduled into a corresponding one of pipeline banks 206, depending on, for example, the memory address of the data being written or requested, request load of pipeline banks 206, or a pseudo-random function. Initial scheduling block 202 schedules read and write requests received from DMC 104, streaming engine 106, PMC 108, and L3 controller 118, by selecting among the first stages of pipeline banks 206. Memory transactions requested to be performed on L3 cache 119 (or exterior functions 120) are arbitrated and scheduled into an L3 cache pipeline by an L3 cache scheduling block 404 in L3 controller 118 (see FIG. 4) after passing through memory pipeline 200 corresponding to L2 cache 116 (pipeline banks 206, and potentially bus snooping-related stages, which are not shown).

Request scheduling prevents conflicts between read or write requests that are to be handled by the same pipeline bank 206, and preserves memory coherence (further discussed below). For example, request scheduling maintains order among memory transactions that are placed into a memory transaction queue (memory access request queue) of initial scheduling block 202 by different memory controllers of the processor 100, or by different bus lines of a same memory controller.

Further, a pipeline memory transaction (a read or write request) sent by DMC 104 or PMC 108 is requested because the memory transaction has already passed through a corresponding level 1 cache pipeline (in DMC 104 for L1D cache 110, and in PMC 108 for L1P cache 112), and is targeted to a lower level cache or memory endpoint (or exterior functions 120), or has produced a miss in the respective level 1 cache, or bypassed L1D cache 110 because a corresponding data payload of a write request is non-cacheable by L1D cache 110. Generally, memory transactions directed to DMC 104 or PMC 108 that produce level 1 cache hits result in a write acknowledgment from L1D cache 110 or a response with data or instructions read from L1D cache 110 or L1P cache 112, respectively. Accordingly, memory transactions that produce level 1 cache hits generally do not require access to the pipeline banks 206 shown in FIG. 2, which control or intermediate memory access to L2 cache 116, L3 cache 119, and exterior functions 120 (see FIG. 1).

Pipeline banks 206 shown in FIG. 2 are part of UMC 114. L1D cache 110 can hold data generated by processor core 102. Exterior functions 120 can access data in L1D cache 110 by, for example, writing the data to L2 cache 116 or L3 cache 119, or using snoop transactions controlled by L3 controller 119 and managed by UMC 114 (L2 controller, as a proxy) to read or evict the data from L1D cache 110.

Memory coherence is when contents (or at least contents deemed or indicated as valid) of the memory in a system are the same contents expected by the one or more processors in the system based on an ordered stream of read and write requests. Writes affecting a particular data, or a particular memory location, are prevented from bypassing earlier-issued writes or reads affecting the same data or the same memory location. Also, certain types of transactions take priority, such as victim cache transactions and snoop transactions.

Bus snooping is a scheme by which a coherence controller (snooper) in a cache monitors or snoops bus transactions to maintain memory coherence in distributed shared memory systems (such as in SoC 10). If a transaction modifying a shared cache block appears on a bus, the snoopers check whether their respective caches have the same copy of the shared block. If a cache has a copy of the shared block, the corresponding snooper performs an action to ensure memory coherence in the cache. This action can be, for example, flushing, invalidating, or updating the shared block, according to the transaction detected on the bus.

“Write streaming” refers to a device (e.g., processor core 102) issuing a stream of write requests, such as one write request per cycle, without stalls. Write streaming can be interrupted by stalls caused by, for example, a full buffer, or by running out of write request identifier numbers. The ability to cause write requests to be pulled from the memory transaction queue as quickly as possible promotes write streaming.

For processor core 102 to know that a write has completed, it must receive a write acknowledgement. To maintain coherence, processor core 102 may self-limit to a given number of outstanding write requests by throttling write requests that would exceed a limit until a write acknowledgement is received for an outstanding write request. Accordingly, processor core 102 and L1D cache 110 may wait on the write acknowledgment (or “handshake”) to proceed, meanwhile stalling corresponding write streaming processes within the processor core 102. Stalls that interrupt write streaming can also be caused by the processor core 102 or DMC 104 waiting for a write acknowledgment from a previous write request. Processor core 102 can also be configured to stall while waiting for a write acknowledgment with respect to certain operations, such as fence operations. Write completion in a lower level cache, such as L2 cache 116 or L3 cache 119, can be detected by DMC 104 (the level 1 cache 110 controller) using a write acknowledgment (handshake) forwarded by UMC 114 (the level 2 cache 116 controller). However, writes can take many cycles to complete, due to various pipeline requirements including arbitration, ordering, and coherence.

At the first level of arbitration performed by initial scheduling block 202, UMC 114 (the L2 cache 116 controller, which includes initial scheduling block 202) determines whether to allow a memory transaction to proceed in memory pipeline 200, and in which pipeline bank 206 to proceed. Writes to L2 cache 116 typically have few operations between (1) initial arbitration and scheduling and (2) write completion. Remaining operations for a scheduled write request can include, for example, checking for errors (such as firewall, addressing, and out of range errors), a read-modify-write action (updating an error checking code of a write request's data payload), and committing to memory the write request's data payload. Generally, each pipeline bank 206 is independent, such that write transactions on pipeline banks 206 (for example, writes of data from L1D cache 110 to L2 cache 116) do not have ordering or coherence requirements with respect to write transactions on other pipeline banks 206. Within each pipeline bank, writes to L2 cache 116 proceed in the order they are scheduled. In the case of partial writes that spawn a read-modify-write transaction, relative ordering is maintained. If a memory transaction causes an addressing hazard or violates an ordering requirement, the transaction stalls and is not issued to a pipeline bank 206. (Partial writes are write requests with data payloads smaller than a destination cache memory's minimum write length. Partial writes trigger read-modify-write transactions, in which data is read from the destination cache memory to pad the write request's data payload to the destination cache memory's minimum write length, and an updated error correction code (ECC) is generated from and appended to the resulting padded data payload. The padded data payload, with updated ECC, is what is written to the destination cache memory.)

Due to these characteristics of the memory pipeline 200, once a write is scheduled within a pipeline bank 206 (for example, a write of data from L1D cache 110 to L2 cache 116), the write is guaranteed to follow all ordering requirements and not to violate coherence (accordingly, conditions to be satisfied to avoid breaking ordering and coherence are met). Committing the write to memory may take a (variable) number of cycles, but a read issued after this write was issued will “see” the write. Accordingly, if the read is requesting data or a memory location modified by the write, the read will retrieve the version of the data or the contents of the memory location specified by the write, and not a previous version. Write-write ordering is also maintained. L3 cache 119 write requests can also be scheduled by the memory pipeline 200 (by UMC 114) so that ordered completion of the L3 cache 119 write requests is guaranteed. These guarantees mean that write requests scheduled into a pipeline bank 206 by the memory pipeline 200 (the L2 cache pipeline) can be guaranteed to comply with ordering and coherence requirements, and to complete within a finite amount of time. Put differently, this guarantee is an assurance that a write transaction that is to a particular address and that is currently being scheduled onto a pipeline bank 206 will “commit” its value to memory (the write will complete and store a corresponding data payload in memory) after a previously scheduled write transaction to the same address, and before a later scheduled write transaction to the same address. This guarantee can be based on the pipeline being inherently “in-order,” so that once a command enters the pipeline, it will be written to memory (commit) in the order it was scheduled. In other words, there are no bypass paths within the pipeline. (The bypass path described below is handled so that it does not break the ordering guarantee, for example, with respect to older transactions targeting a same memory address.)

“Contemporaneously with” is defined herein as meaning at the same time as, or directly after. Accordingly, a first event occurring “contemporaneously with” a second event can mean that the two events occur on the same cycle of a system clock.

UMC 114 (the L2 controller) sends the write acknowledgment for writes of data to L2 cache 116 or higher level cache (for example, of data from L1D cache 110) to DMC 104 (the L1 controller) contemporaneously with the corresponding write request being scheduled by initial scheduling block 202 (the first level of arbitration). Accordingly, the write acknowledgment indicating write completion is sent contemporaneously with the write request being scheduled, rather than after memory pipeline 200 finishes processing the write request. This accelerated acknowledgment is enabled by the guarantee that the scheduled write request will complete in order and in compliance with coherence requirements. UMC 114 gives the illusion that write requests are being completed on the cycle on which they are scheduled, rather than the cycle on which corresponding data is committed (written) to memory. From a perspective of observability of processor core 102 or DMC 104, it is as if L2 cache 116 instantly completed the write request when the write request was scheduled. This enables DMC 104 to un-stall processor core 102 more quickly (or prevent processor core 102 stalls), and enables write requests to be pulled from queue with lower latency (faster), improving overall performance. The queue is a queue of transactions in UMC 114 that are sent from respective “masters” (functional blocks that can send memory transactions to UMC 114 to be queued), such as DMC 104, streaming engine 106, PMC 108, MMU 117, and L3 controller 118. The queue can be implemented as holding stages where memory transactions reside while waiting to be arbitrated and scheduled into a pipeline bank 206 by initial scheduling block 202.

Processor core 102 generally is configured to read data from memory to work on the data. This is also true for other processor cores 102 of other processors (such as processors 100) of SoC 10, with respect to memory accessible by those other processors. However, other processors of SoC 10 require data generated by processor core 102 to be available outside the data-generating processor 100 to be able to access the generated data. This means the generated data passes through L3 controller 118 to be externally accessible, either within shared memory (L3 cache 119) or by transmission to exterior functions 120.

FIG. 3 is an example of a process 300 for memory write operations for SoC 10 of FIG. 1. In step 302, initial scheduling block 202 schedules a write request, directed to L2 cache 116, to a pipeline bank 206. In step 304, directly after the write request is scheduled to the pipeline bank, UMC 114 (L2 cache controller) sends a write acknowledgment to processor core 102. In step 306, processor core 102 is un-stalled in response to the write acknowledgment.

FIG. 4 is a block diagram of an example memory pipeline 400 for SoC 10 of FIG. 1. Memory pipeline 400 as shown in FIG. 4 is similar to a memory pipeline 200 as shown in FIG. 2, but also includes a bypass path 402. Bypass path 402 couples initial scheduling block 202 to L3 cache scheduling block 404, so memory pipeline corresponding to at least one level of cache is skipped. For example, bypass path 402 enables bypass writes to bypass portions of memory pipeline 400 related to L2 cache 116, shortening the total processing time for bypass writes. Accordingly, writes to certain memory regions can be written to L3 cache 119 without being written to L2 cache 116 (or other lower level cache). Such writes (bypass writes) can be safely performed without access to memory pipeline stages related to L2 cache 116, including pipeline banks 206 and related bus snooping (not shown). This simplifies coherence requirements for bypass writes.

Memory coherence imposes ordering requirements in order for bypass writes to be allowed to use bypass path 402. For example, writes may not bypass other writes (write-write ordering); writes may not bypass reads (read-write ordering); writes may not bypass victim cache transactions (for example, write requests from L1D victim cache 113 to L1D cache 110, which can be caused to be processed by UMC 114 by a cache miss of the victim cache-related write request at the L1 level); and writes may not bypass snoop responses (for example, snoop responses corresponding to a controller requesting a write from victim cache to L1D cache 110 when the request is not caused by a cache miss). Victim cache transactions and snoop responses are high priority because they constitute memory synchronization events. Also, L1D cache 110 victims go through the full pipeline because, for example, they include updating an internal state in UMC 114. However, a bypass write directly following a victim cache transaction can also be prioritized, ensuring that the bypass write will not be blocked or stalled (analogous to slip-streaming). This prioritized status applies solely to the single bypass write directly following a victim cache transaction, and ends when the bypass write has been sent to the L3 controller 118 for further processing.

Initial scheduling block 202 may have a designated a bypass state (or “bypass mode”) in which bypass writes can be scheduled to bypass path 402, rather than to the full memory pipeline 400 (including to a pipeline bank 206). When initial scheduling block 202 is in bypass mode, bypass writes bypass the entire pipeline of an intermediate level of cache, including associated internal arbitration. When initial scheduling block 202 is not in bypass mode, bypass writes go through the full memory pipeline 400.

FIG. 5 is an example of a process 500 for memory write operations for SoC 10 of FIG. 1. FIG. 5 describes conditions for initial scheduling block 202 to enter and remain in bypass mode, so that bypass writes are allowed to proceed along the FIG. 4 bypass path 402. In step 502, initial scheduling block 202 determines whether the next memory transaction in the queue is a bypass write, carrying data or relating to a memory location guaranteed not to be written to L2 cache 116. (For example, a data payload too large to fit in the L2 cache 116 or containing a type of data not stored by the L2 cache 116.) If not, then the next memory transaction is processed using normal (non-bypass) processing (goes through the full memory pipeline 400) according to step 504, and the process returns to step 502. In step 506, initial scheduling block 202 determines whether a write to or from L1D victim cache is in the pipeline (in a pipeline bank 206). If so, the victim cache write is prioritized as a memory synchronization event, and in step 504 the bypass write is not allowed to proceed along bypass path 402. Instead, the bypass write receives normal (non-bypass) processing (albeit prioritized non-bypass processing), and process 500 returns to step 502. In step 508, initial scheduling block 202 determines whether there are L1D writes (writes from DMC 104; in some embodiments, any write requests generated outside the memory pipeline 200) in a pipeline bank 206 (the pipeline bank 206 the bypass write would be scheduled to). If so, in step 510, the bypass write is delayed until the L1D writes (all L1D writes in the corresponding pipeline bank 206; in some embodiments, all write requests generated outside the memory pipeline 200) are cleared from respective pipeline banks 206 (for example, respective writes are committed to memory, or reach L3 cache scheduling block 404 for scheduling into a corresponding memory pipeline of the L3 controller 118). Otherwise, in step 512, the bypass write enters bypass mode 514, and the bypass write is sent to L3 controller 118 (and from there to L3 cache 119) along bypass path 402. Next, in step 516, initial scheduling block 202 checks whether the next memory transaction in its queue is a bypass write. If so, process 500 remains in bypass mode by returning to step 512, and the next memory transaction in the queue (a bypass write) is sent to L3 cache scheduling 404 along the bypass path 402. Accordingly, sequentially queued bypass writes after a bypass write has fulfilled conditions to use bypass path 402 can also be sent to L3 cache scheduling 404 via bypass path 402 without having to re-check conditions addressed in steps 506 and 508. If the next memory transaction in the queue of initial scheduling block 202 is not a bypass write, then in step 518 initial scheduling block 202 exits bypass mode, the next memory transaction in the queue receives normal (non-bypass) processing, and the process 500 returns to step 502.

Modifications are possible in the described embodiments, and other embodiments are possible, within the scope of the claims.

In some embodiments, the streaming engine passes on and returns responses for both read and write requests.

In some embodiments, the processor can include multiple processor cores (embodiments with multiple processor cores are not shown), with similar and similarly-functioning couplings to DMC, the streaming engine, and PMC to those shown in and described with respect to FIG. 1.

In some embodiments, bus lines enabling parallel read or write requests can correspond to different types of read or write requests, such as directed at different blocks of memory or made for different purposes.

In some embodiments, the streaming engine enables the processor core to communicate directly with lower level cache (such as L2 cache), skipping higher level cache (such as L1 cache), to avoid data synchronization issues. This can be used to help maintain memory coherence. In some such embodiments, the streaming engine can be configured to transmit only read requests, rather than both read and write requests.

In some embodiments, L3 cache or other lower level memory can schedule write requests so that a write acknowledgment can be sent to DMC (or the processor core or another lower level memory controller) contemporaneously with the write request being scheduled into a corresponding memory pipeline.

In some embodiments, different memory access pipeline banks can have different numbers of stages.

In some embodiments, processors in exterior functions can access data stored in L2 cache; in some such embodiments, coherence between what is stored in L2 cached and what is cached in other processors in exterior functions is not guaranteed.

In some embodiments, writes can be bypass writes if included data is too large for lower level cache (such as L1D cache or L2 cache).

In some embodiments, a write can be a bypass write if a page attribute marks the write as corresponding to a device type memory region that is not cached by UMC (the L2 cache controller).

In some embodiments, L1D cache (or other lower level cache) can cache a data payload of a bypass write.

In some embodiments, memory coherence rules of a processor forbid bypassing memory transactions (memory read requests or memory write requests) other than or in addition to L1D victim cache writes and L1D writes.

In some embodiments, the bypass path jumps a bypass write to a final arbitration stage prior to being scheduled to enter a memory pipeline bank of the L3 cache (not shown).

In some embodiments, a guarantee that a memory write will never be directed to L2 cache (corresponding to a bypass write) includes a guarantee that this this is a choice will never change, or that such a write to L2 cache is impossible.

In some embodiments, a guarantee that a memory write will never be directed to L2 cache includes that L2 cache does not have a copy of, or a hash of, corresponding data.

In some embodiments, a guarantee that a memory write will never be directed to L2 cache can change (be initiated, where the guarantee was not recently in force). In such embodiments, newly making this guarantee (for example, changing a corresponding mode register to make the guarantee) while a line is being written to L2 cache can require that corresponding L2 cache be flushed. A cache flush in this situation avoids a cached copy of a data payload now guaranteed not to be written to L2 cache remaining in L2 cache after the guarantee is made.

In some embodiments, cache controllers only originate memory transactions in response to transactions originated by processor core 102 or exterior functions 120.

In some embodiments, a read or write request can only be scheduled into a corresponding one of pipeline banks 206, depending on, for example, the memory address of the data being written or requested, request load of pipeline banks 206, or a pseudo-random function. 

What is claimed is:
 1. A processor system comprising: a processor core configured to generate memory transactions; a first level cache memory; a first memory controller coupled to control the first level cache memory; a second level cache memory that is a different level cache memory than the first level cache memory; and a second memory controller having a memory pipeline configured to process the memory transactions, the second memory controller coupled to control the second level cache memory, the second memory controller connected to the first memory controller by a bypass path that skips the memory pipeline, the second memory controller configured to: determine whether a first memory transaction to be scheduled is a bypass memory write request, a bypass memory write request being a memory write request having a data payload or corresponding to a memory location that is indicated not to result in a corresponding write being directed to the second level cache memory; and if the first memory transaction is a bypass memory write request, determine whether a second memory transaction in the memory pipeline prevents passing by the first memory transaction, and if not, send the first memory transaction to the first memory controller using the bypass path.
 2. The processor system of claim 1, wherein the second memory controller is configured to: if the first memory transaction is sent to the first memory controller using the bypass path, enter a bypass mode; and if a third memory transaction sequentially subsequent to the first memory transaction is determined to be a bypass memory write request, and the second memory controller is in the bypass mode, skip the determination of whether a second memory transaction prevents passing action, and send the third memory transaction to the first memory controller using the bypass path.
 3. The processor system of claim 2, wherein the second memory controller is configured to, if the third memory transaction is not determined to be a bypass memory write request, exit the bypass mode.
 4. The processor system of claim 1, wherein the second memory controller is configured not to cause the data payload of the first memory transaction to be written to the second level cache memory after sending the first memory transaction to the first memory controller using the bypass path.
 5. The processor system of claim 1, wherein the first memory controller is configured to process the first memory transaction after receiving the first memory transaction through the bypass path.
 6. The processor system of claim 1, further comprising: a third level cache memory that is a different level cache memory than the first level cache memory and the second level cache memory; and a victim cache of the third level cache memory; wherein the second memory controller is configured so that memory write requests that request memory writes from the victim cache to the third level cache memory are memory transactions that prevent passing by the first memory transaction.
 7. The processor system of claim 6, wherein if a memory write request requesting a memory write from the victim cache to the third level cache memory is determined to be in the memory pipeline when the first memory transaction is a bypass memory write request, the bypass memory write request is scheduled into the memory pipeline and performed by the memory pipeline.
 8. The processor system of claim 1, further comprising a third level cache memory that is a different level cache memory than the first level cache memory and the second level cache memory; wherein the second memory controller is configured so that memory write requests that request writes to the third level cache memory are memory transactions that prevent passing by the first memory transaction.
 9. The processor system of claim 8, wherein the second memory controller is configured to delay the first memory transaction until the determination of whether a second memory transaction prevents passing action determines that there is no third level cache write request in the memory pipeline, and after that to send the first memory transaction to the first memory controller using the bypass path.
 10. The processor system of claim 1, wherein the bypass path is directly connected from a memory transaction scheduling portion of the first memory controller to the second memory controller.
 11. A method of operating a processor system, wherein the processor system has a first level cache memory and a second level cache memory, the second level cache memory being a different level cache memory than the first level cache memory, the first level cache memory having a first memory controller and the second level cache memory having a second memory controller, the method comprising; determining, using the second memory controller, whether a first memory transaction is a bypass memory write request, the first memory transaction a memory transaction to be scheduled by the second memory controller, a bypass memory write request being a memory write request having a data payload or corresponding to a memory location that is indicated not to result in a corresponding write being directed to the second level cache memory; determining whether a memory transaction that prevents passing is in a memory pipeline of the second memory controller; and sending the first memory transaction from the second memory controller to the first memory controller using a bypass path that skips the memory pipeline of the second memory controller, the sending performed based on determining the first memory transaction is a bypass memory write request and based on determining no memory transaction that prevents passing is in the memory pipeline.
 12. The method of claim 11, wherein the bypass path is directly connected from a memory transaction scheduling portion of the second memory controller to the first memory controller.
 13. The method of claim 11, further comprising: entering a bypass mode in the memory pipeline, based on the sending step having been performed on the first memory transaction.
 14. The method of claim 13, further comprising: determining, using the second memory controller, whether a second memory transaction is a bypass memory write request, the second memory transaction a memory transaction to be scheduled by the second memory controller; not determining whether a memory transaction that prevents the second memory transaction from passing is in the memory pipeline of the second memory controller, based on determining the second memory transaction is a bypass memory write request and based on the memory pipeline being in the bypass mode; performing the sending step on the second memory transaction, based on determining the second memory transaction is a bypass memory write request and based on the memory pipeline being in the bypass mode; and exiting the bypass mode and processing the second memory transaction using the memory pipeline, based on determining the second memory transaction is not a memory write request and based on the memory pipeline being in the bypass mode.
 15. The method of claim 11, further comprising: the second memory controller not causing a data payload of the first memory transaction to be written to the second level cache memory after sending the first memory transaction to the first memory controller using the bypass path.
 16. The method of claim 11, further comprising: processing the first memory transaction using the first memory controller after the first memory controller receives the first memory transaction through the bypass path.
 17. The method of claim 11, wherein the processor system has a third level cache memory that is a different level cache memory than the first level cache memory and the second level cache memory, the third level cache memory having a victim cache; and wherein victim cache memory-related memory transactions prevent passing by the first memory transaction; the method further comprising processing the first memory transaction using the memory pipeline based on determining a victim cache memory-related memory transaction is in the memory pipeline.
 18. The method of claim 11, further comprising: processing the first memory transaction using the memory pipeline, based on determining the memory transaction is not a bypass memory write request.
 19. The method of claim 11, wherein the processor system has a third level cache memory that is a different level cache memory than the first level cache memory and the second level cache memory; and wherein memory write requests that request memory writes to the third level cache memory are memory transactions that prevent passing by the first memory transaction.
 20. The method of claim 19, further comprising: delaying the first memory transaction, using the second memory controller, based on determining the first memory transaction is a bypass memory write request and based on determining a memory write request that requests a memory write to the third level cache memory is in the memory pipeline; and after performing the delaying step, performing the sending step on the first memory transaction based on determining the memory write request that requests the memory write to the third level cache memory is no longer in the memory pipeline. 